vu meter help

[cowanrg]

i was at my local electronics surplus shop today and picked up a couple VU meters (they were new and $2, so why not, i need some for the alephs anyways).

well, they have a scale that on the top reads 1 - 100, then to the right of that, a red scale. the bottom readings are -20 to -1, then in the red zone, its +1, +2, and +3.

the brand is modutec, but i dont see a model # and couldnt find one on a website. does anyone know how these things work, or what they measure? can i use it? if not, someone can buy them off me, or i can return them.

edit:

i looked on digikey, and one JUST like it (cant tell if its the same because i dont have this model #), sells for $29 each. so a good deal if i cant get them to work.

[Mr Evil]

I have a circuit for driving a VU meter like the one you describe on my site (under electronics->VU meter), built from information gleaned from this page..

They're pretty simple really: They measure the level of an audio signal, calibrated in a way that is designed to correspond to our subjective perception of loudness. Drive them by rectifying and smoothing the audio signal.

[cowanrg]

this is simply for cosmetics and i dont care about accuracy, i just want a bouncing needle that goes to the music.

it looks like the first example, (figure 1), can be made with a diode bridge and a cap? if this is the case, i can toss one of those together really quick.

[Mr Evil]

In that case you could go even simpler and use a single diode rectifier, as often seen in cheap tape decks.

[cowanrg]

ah ok. so it just wants to see a DC feed then? not ac?

[Mr Evil]

Yeah, just DC.

[cowanrg]

ok cool.

how do i trim or increase the input? i assume i can trim it by changing the resistor value, but can i increase the input, so its MORE sensitive.

[Mr Evil]

A full-wave rectifier will be more sensitive than a single diode. If you need more than that then you'll have to use some sort of gain stage... probably an op-amp or transistor. Or you might be able to use a small transformer if the source can handle the load.

It might not be necessary though. The ones I have show full scale deflection at a mere 240mV, which a line level input should be able to provide.

[cowanrg]

i plugged it in, using a 600V bridge rectifier i had laying around, and it appeared to work fine. it was maybe a bit TOO sensitive. so a guess a resistor could help.

does this circuit, being that it's in the signal path hurt the music any amount? if i throw a whole circuit in there im sure it could, but with just a resistor, the meter and the rectifier, how bad can it hurt it? (im planning on maybe having a switch that switches it in and out of the signal path).

[Mr Evil]

That depends on how good the source is. The current drawn will be non-linear, which could theoretically cause problems, but I don't think it would matter in most cases. You'll have the chance to test it, so you can tell me!